fix: restore watermark image in email templates

The previous commit incorrectly removed the watermark — only the
style extraction into <style> blocks was requested. Restores the
watermark <img>, {{watermarkUrl}} placeholder resolution in both
EmailConnectorService and PasswordResetNotificationService, and
the corresponding test assertions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/test/java/net/siegeln/cameleer/saas/vendor/EmailTemplateLoadingTest.java

@@ -45,6 +45,25 @@ class EmailTemplateLoadingTest {
         }
     }
 
+    @Test
+    void templatesContainWatermarkPlaceholder() throws IOException {
+        for (String path : TEMPLATE_FILES) {
+            String content = new ClassPathResource(path).getContentAsString(StandardCharsets.UTF_8);
+            assertTrue(content.contains("{{watermarkUrl}}"),
+                path + " must contain {{watermarkUrl}} placeholder");
+        }
+    }
+
+    @Test
+    void watermarkPlaceholderIsReplaced() throws IOException {
+        String content = new ClassPathResource("email-templates/register.html")
+            .getContentAsString(StandardCharsets.UTF_8);
+        String resolved = content.replace("{{watermarkUrl}}",
+            "https://example.com/platform/assets/email-watermark.png");
+        assertFalse(resolved.contains("{{watermarkUrl}}"));
+        assertTrue(resolved.contains("https://example.com/platform/assets/email-watermark.png"));
+    }
+
     @Test
     void templatesContainBrandElements() throws IOException {
         for (String path : TEMPLATE_FILES) {